1. Field of the Invention
This invention relates to an apparatus for measuring the thickness of a material such as a film which has been coated on a surface of a film-like base.
2. Description of the Prior Art
In the manufacturing processes for coating a film on a film-like base, it is necessary to inspect the thickness of the coating of film which has been applied to the base. As apparatuses for measuring the thickness of a material such as a coating of film, an apparatus wherein light is irradiated to the material and the amount of light reflected from the material is measured, an apparatus wherein a plurality of light beams having different wavelengths are irradiated to the material and the difference in reflectivity between the light beams is measured, and the like have heretofore been used.
However, with the conventional techniques described above, the measurement sensitivity is not substantially high and the measured values fluctuate largely when the conventional techniques are used in processes wherein a high accuracy is required in the measurement of the thickness of a coating of film. Also, a very large error in measurement may arise, depending on the kind of substance used to form the film coating. Therefore, the conventional techniques are not applicable to many kinds of film coatings.
In order to eliminate the aforesaid problems in the conventional techniques, a novel method for measuring the thickness of a material has been proposed in Japanese Unexamined Patent Publication No. 57(1982)-52806. The proposed method comprises the steps of separating light emitted by the material into its spectral components, carrying out colorimetry in order to find chromaticity coordinates, calculating the gradient of a straight line which connects a coordinate point having the chromaticity coordinates on the chromaticity diagram to a standard coordinate point having standard coordinates, and finding the thickness of the material on the basis of the value of the gradient.
However, with the method wherein chromaticity coordinates are found in order to find the thickness of the material, because the gradient of the straight line which connects the coordinate point having the chromaticity coordinates to the standard coordinate point is calculated, it is necessary to calculate two or more variables in order to specify a single coordinate point. For example, in order to specify the coordinates of a measurement point in an X-Y coordinate system, it is necessary to calculate the values of variables X and Y representing the coordinates. Therefore, it takes a long time to find the thickness of the material, and a memory having a large capacity is required to store information tables.
Further, there has been known an optical thickness measuring apparatus in which light which has been separated into its spectral components and has been reflected by a material is received by a photodetector, variable b.sup.* of CIE L.sup.* a.sup.* b.sup.* colorimetric system or the reflectivity pattern for each wavelength is calculated on the basis of information about the light detected by the photodetector, and the thickness of the material is calculated from the value of variable b.sup.* calculated or the reflectivity pattern calculated. {See U.S. Pat. No. 4,785,336 (McComb et al), Japanese Patent Application No. 59(1984)-87307 (Yumiba), and the like.}
However, in these conventional techniques, since an integrating sphere is used to collect the light, the scatter-reflected light and the regular-reflected light are detected simultaneously. In magnetic tape or the like which has a high surface gloss, the intensity of the regular-reflected light on its surface is too high to be influenced by the thickness of the layer.
Accordingly, it is very difficult for the system of those conventional techniques mentioned above to measure, in high degree accuracy, the thickness of the layer of magnetic tapes or the like.